I mean, obviously the answer is Accel (dude says Henshin like he's passing a Gaia Memory), but just to have some other opinions, lemme stump for the laid-back dark reflection of Ryuki, Ryuga. I really like the way he doesn't shout "HENSHIN!", he just says "Henshin." It's somehow more terrifying when someone treats the Henshin less as a primal will-to-power, and more as a Well You Just Missed Your Last Chance To Avoid Me Killing You, by-the-by sort of thing.

nobody's even mentioned Kenzaki yet smh

Oh wait I remembered Beast's big long drawn out "Heeeeeeeennnnnnnnnn-shin!" I change my vote to Mr Mayonnaise.

Part of me wonders if Terui shouting henshin is to intimidate his enemies because they'd be like "Shit..... he looks pissed I picked the wrong opponent to mess with"

I'll join th club of people who immediately thought of Terui after reading the thread title

Terui's is really iconic, it keeps the aggressive energy of riders who just shout it but is more distinct. I always wondered if he made that pause to emphasize "shin" as in death.